Lignin is a complex polymer occurring in certain plant walls making the plant rigid. Bonds lignin to cellulose are broken during a chemical pulping process. Lignin isolation from black liquor has been used during past years to provide lignin for commercial use, for example for use as a solid biofuel and dispersant. This lignin is also a valuable material for production of “green chemicals” and as a fuel for the production of chemicals. The production process of lignin of that kind is described for example in WO2006/031175. According to the process, lignin is separated from black liquor. The separation method may include steps to acidify the black liquor so that the lignin is precipitated. The solid phase is then separated from the liquor and can thereafter be cleaned or modified.
However, there is a desire to use lignin products also in other applications than fuel applications. The lignin product obtained by the isolation process is a renewable, non-poisonous environmentally friendly product which could be used for example as a raw material for building materials. However, the obtained lignin product suffers from a drawback of being malodorous, whereby the use of the product has been limited to few applications. There is thus a great desire to reduce or eliminate the problems with odor in lignin products.
In the prior art, there have been attempts to reduce odour levels in lignin products. WO2012161865 discloses a method in which pressurized black liquor may be reacted with an oxidizing agent, such as oxygen, peroxide or the like, in an amount sufficient to reduce or eliminate the odor level in the black liquor so that there will be little or no odor in the final lignin product. This step removes the odors by oxidating mercaptans (methyl, ethyl), and dimethyl, diethyl sulphides etc. However, with this process there is a risk that also lignin is oxidized and thus deteriorated or chemically modified.
Even though there are prior art solutions for the reduction of odor levels, especially in respect of mercaptans, there is still a need for a process that removes other organic odorous compounds effectively. There is also a need for a process in which lignin is affected as little as possible and in which lignin is not essentially fractionated by the odour reduction process. There is also need for an environmentally friendly process with a reduced risk for hazards in the production process. There is also a need for a process which can be integrated with the present lignin separation processes in a simple way. Further there is a need to use environmentally friendly products throughout the process for the isolation of lignin.